The present invention relates to photolithographic processing systems and, more particularly, to a system for designing and using photomasks for use in semiconductor processing technology.
Thin film technology has been used for electrical interconnection of integrated circuits and discrete components. This thin film technology involves the use of high conductivity metal films that are deposited on a substrate. The term "thin film" is used to describe approximate film thickness of 0.0002 inch or less, compared to the larger geometry and thicker films associated with hybrid integrated circuits. In addition to providing electrical interconnection of integrated components, thin film circuits are also used to form resistor or capacitor structures on the substrate by deposition of resistive or dielectric thin film layers.
Thin film technology is particularly well suited to high speed systems which require a high packaging density. The use of thin film technology for these systems, instead of the more traditional printed wiring board technology (PWB), provides reduced interconnection distances between the integrated components and low capacitance interconnects, both of which enhance the system performance. A general discussion of the use of thin film technology is found in the article entitled "Multi-chip Modules for High Performance Military Electronics," Electrecon '91 Proceedings sponsored by the Electronics Manufacturing Productivity Facility, Indianapolis, Ind., Oct. 27 and 23, 1991, incorporated herein by reference.
Thin film processing technology for transforming a substrate into an electrical interconnection or module for integrated circuits and discrete components involves a complex sequence of process steps which must be accurately performed to produce a high yield of these large and relatively expensive modules. Thin film processing technology involves photolithography steps which are utilized to produce patterned masking layers on the substrate which in turn are utilized to create different layers of conductive film, resistors, capacitors, and vias for interconnecting different layers of conducting film. A typical thin film process involves a number of sequential photolithography operations, each of which must be performed in its proper sequence and with proper process control to produce a good thin film module. If one of the photomasks is utilized out of sequence, the thin film module will be defective.
As discussed previously, thin film technology frequency uses multiple layers of thin film conducting material. These thin film conducting layers are separated by a dielectric material such as polyimide. Connections between various layers of conductive film are accomplished with a vertical interconnection frequently referred to as a "via." These vias are frequently formed by defining the via locations using a photomask and plating a conductive material, thereby forming the vertical interconnection to selected conductive layers that are aligned with this via. The method of forming vias is known and therefore will not be discussed here in detail. However, it is apparent that the alignment of the conductive films at various layers in the module is essential to via formation. If any layer of conductive film is misaligned relative to any other layer of conductive film, the via will not be properly formed at the correct location and therefore an electrical interconnection will not occur when the via is plated with a conductive material. Therefore, the alignment of each photomask that is used in the photolithographic process is essential to ensure that each conductive layer is properly aligned so that the electrical interconnection occurs at correct locations.
Previous methods used to align these photomasks have employed an alignment cross on each photomask. A cross pattern is deposited onto the structure during the same deposition forming the mask feature. The photomask for the next step, and each subsequent processing step, is aligned by positioning that mask's cross over the cross structure defined by the alignment cross of the previous photomask. A slight misalignment occurs in a lower layer of the structure, the alignment mask is blurred and subsequent masks are often misaligned, forming structures that are misaligned. In addition, the dimension of the cross structure formed by the processing steps is frequently larger in dimension than the corresponding cross on the photomask because of growth. Growth is a known phenomenon of the deposition process that results in the structure defined by the photomask having larger dimensions than the corresponding structures on the photomask. As a result of this growth, the alignment of photomasks becomes more difficult because the cross on the photomask must be aligned with each of the prior cross structures that are not only possibly slightly misaligned, but also may be larger due to growth.
In addition to growth and slight misalignment of the mask, there are other problems that make it difficult to align a series of masks each of which has a cross structure that is superimposed. As previously discussed, a dielectric material is placed over each conductive layer to insulate that conductive layer or film from the next sequential conductive layer or film. This dielectric material is frequently cured by baking which often times causes shrinking or contraction. As this dielectric material shrinks, the entire substrate may tend to warp or bow. Because the patterns of conductive film of each layer adds some stress to the substrate, and because these patterns of conductive film at various layers are not uniform, the stress acting on the substrate as a result of the dielectric curing is also non-uniform thereby causing substrate bowing or warping. Warping or bowing of the substrate changes the flatness of the substrate relative to the mask resulting in changes in spacing between alignment marks on the mask and patterns of conductive film defined by the mask. The changes in spacing due to warping tends to blur cross structures defined by previous masks thereby making mask alignment more difficult.
There is a present need for mask alignment systems for aligning a sequence of photomasks with the structures produced by the previous photomasks. The ideal mask alignment system should enable an operator to align masks even if substrate bowing or warping occurs. In addition, an ideal mask alignment system should allow mask alignment personnel to readily determine the mask sequence, thereby preventing a mask layer from either being missed or being used out of order. Finally, this ideal mask alignment system should permit subsequent quality review of each mask alignment of the process.